Soil sodicity - Soil Essentials: Managing Your Farm's Primary Asset - page 63

weakens the bonds between the soil particles when wetted, resulting in clay
swelling and dispersing (particles becoming detached). The dispersed clay particles
can move through the soil, clogging soil pores. Both swelling and dispersion
decrease the permeability of soils, thus reducing infiltration, aeration and
drainage. The broken down soil particles in a sodic soil can be moved easily by
water or wind erosion.
Solid phase . Soil particles, including organic matter, clay, silt, sand and gravel,
mixed together to form a natural medium in which most plants can grow.
Solution phase . The aqueous phase of the soil and its solutes consisting of
soluble materials and ions dissociated from the surfaces of the soil particles.
Figure 22 shows how a saline soil becomes a sodic soil.

Types of sodicity

Inherent sodicity:
Soil sodicity resulting from the natural addition of sodium from cyclic salt and
leaching of calcium displaced from clay colloids in saline soils (a long-term
process).

Induced sodicity:
Soil sodicity resulting from the use of high sodium content irrigation waters (bore
water or eff luent) or as a result of sodic subsoil clay being brought to the surface or
exposed by land forming (short-term process).

Legend
NaCl Sodium chloride (salt)

Rain Irrigation

Na Sodium

Cl Chloride

Ca Calcium

NaCl NaCl NaCl NaCl -Na -Na -Na -Na

Clay particles

-Ca -Ca -Ca -Ca

Cl Cl Cl Cl

NaCl NaCl NaCl NaCl

NaCl NaCl NaCl NaCl

Ca Ca Ca Ca

Salts, chloride and calcium are
washed down into the soil
leaving sodium in the surface
layers bound to clay particles.

SALINE SOIL SODIC SOIL

Figure 22 How a saline soil becomes a sodic soil. ( Soil Sense C-12)